I think that this example that you propose can be exposed for any type of modification.
Before trying to color my first image I thought that illustrator or Photoshop were complex tools to use and when I painted the first image I thought "Well it was really easy", of course the first coloring compared to the most current ones is quite mediocre but for an initial job where I had no knowledge of how to do it seemed quite "easy"
As I progressed, I learned new things and I have improved in many aspects and I really feel that every day you can learn more and more things.
I think to answer your question why hasn't anyone done it before? I would say it's because these types of projects are very time consuming and require commitment, not everyone wants to sit down for 2 to 4 hours to work for something for which they won't receive money. .
For my part I have received some messages from people who want to help me with the coloring, but for the images to be as similar as possible I would have to teach them from scratch all the techniques I use and once I tried it with a person...
Result: I lost all communication with that person, which made me think that I wasted a lot of time
For this reason, whenever someone tells me that they want to help me in some way, I reject the request. It may sound a bit arrogant, but I really don't want to risk it since I don't know how much commitment that person will have.
I think that for this same reason, Inusuku explicitly says on his twitter "I work alone", I think that similar situations have also happened to him.
For these reasons, more than anyone else, I appreciate the work that everyone does: Blue Akapp Mello Shade.
And I always try to encourage them to continue because we all have something in common, the love we have for this game.